Apocalypse

On the level/player selection screen, move the mouse to the crystal at the
top of the Apocalypse logo. Imagine mouse buttons numbered like this:
SELECT 1
MENU 2
ADJUST 4
Holding down combinations of buttons activates a different numbered
cheat mode, according to the sum of the held down buttons. The cheat mode
levels run from 1 to 7, but 5 to 7 are the same. As the cheat level gets
higher so the better the cheats become. On all cheat levels you can select
any planet to start at. At cheat levels 3 and higher the shield charge
increases, at levels 3 andhigher you have rapid fire, and the gun
temperature cooler improves in cheat levels up to 4. It is a bit tricky to
select any particular level, so a way to get cheat level 5 (the best) is
provided, which is to press [Z] to continue instead of Space when you
insert the levels disc.

Level skip: Independant of the above cheat, during the game hold down
[Shift] and type in ELIZA to go to the next level. (Information from Cheat Machine)